doc: added help on 'clean' command to remove build

README.rst

@@ -70,6 +70,10 @@ You can build recipes at the same time by adding them as parameters::
 
     $ ./toolchain.py build openssl kivy
 
+Recipe builds can be removed via the clean command e.g.::
+
+    $ ./toolchain.py clean openssl
+
 The Kivy recipe depends on several others, like the sdl* and python recipes.
 These may in turn depend on others e.g. sdl2_ttf depends on freetype, etc.
 You can think of it as follows: the kivy recipe will compile everything